In a vehicle such as an automobile, a transmission is disposed between an internal combustion engine and a driving wheel. The transmission varies a driving force transmitted to the driving wheel from the internal combustion engine and a traveling speed in a manner to meet with traveling conditions of the vehicle which vary in a wide range, so that the internal combustion engine can exhibit satisfactory performance. There are several types of transmissions, for example a gear type transmission in which a driving force is transmitted by changing the gear ratio in steps by means of selectively switching the meshing state of a multistage gear train, and for example a continuously variable transmission having a fixed pulley piece and having a movable pulley piece which is supported for movement toward and away from the fixed pulley, the width of a groove between the pulley pieces being increased or decreased in order to increase or decrease the radius of rotation of a belt looped around a driving side pulley and a driven side pulley which include the fixed and movable pulley pieces to thereby continuously vary a gear ratio to transmit a driving force. Such a continuously variable transmission is disclosed, for example, in Japanese Patent Early Laid-open publication No. Sho 57-186656, Japanese Patent Early Laid-open Publication No. Sho 59-43249, Japanese Patent Early Laid-open Publication No. Sho 59-77159, and Japanese Patent Early Laid-open Publication No. Sho 61-233256.
As a continuously variable transmission which varies the belt ratio by hydraulic pressure, one is known which is provided with a hydraulic clutch which is engaged and disengaged by clutch pressure in order to adjust a driving force output by this continuously variable transmission. This continuously variable transmission, even when a vehicle is stopped, in the case where the gear is for example shifted into a forward position, effects such a degree of clutch control pressure as is just enough to cause the hydraulic clutch slightly engaged in its hold mode in order to cause the sliding hydraulic clutch to transmit a very weak driving force to thereby generate a small amount of creep. Owing to this creep, starting on a slope is facilitated.
If this creep is large, the vehicle moves when the vehicle is stopped to wait for a traffic signal or the like in a state where the gear is shifted to its forward position. Therefore, it is necessary to perform an operation such as pressing a foot brake down hard, actuating a side or parking brake, or shifting the gear into its neutral or parking position. Furthermore, if the creep is large, vibrations tend to occur and driving performance is jeopardized. In view of the foregoing, the clutch pressure was heretofore set so that the creep would become small when the vehicle was running in its normal mode in order not to jeopardize the driving performance when the gear is shifted into its forward position.
However, in the case where that continuously variable transmission was operated in its reverse position, if the creep is small, it is difficult to drive with much care when the vehicle is moving in reverse. As far as driving performance is concerned, the creep should be large. That is, if the creep is small, it becomes difficult to produce reverse movement of the vehicle only by means of creep, and it is necessary to step down on the accelerating pedal in order to move the vehicle backward. Because of the foregoing reason, in the conventional continuously variable transmission, when the accelerating pedal is stepped on in order to effect reverse movement of the vehicle after the gear is shifted into its reverse position, a clutch pressure of the hydraulic clutch is brought into a normal start mode in which it is stronger than in the hold mode, or otherwise it becomes a drive mode clutch pressure. However, it is difficult to adjust the speed to a level that the driver desires when in a normal start mode, and the reverse vehicle speed becomes too fast. Therefore, it is not easy for the driver to drive the vehicle backward.
It is therefore an object of the present invention to provide a creep control for a continuously variable transmission, in which driving performance is improved when the continuously variable transmission is operated in reverse without jeopardizing driving performance when the continuously variable transmission is operated in forward and, moreover, in which this control can be added to a conventional continuously variable transmission with ease.